In digital electronics, predetermined voltage levels represent one of two possible states at any point in a circuit. These states, HIGH and LOW, correspond to the "0" (false) and "1" (true) states of Boolean logic. Precise voltages are not necessary in digital electronics because all that is necessary is have two voltages that are distinguishable to represent the HIGH and LOW states.
There are three popular varieties of digital electronics. These are complementary metal oxide semiconductor ("CMOS"), transistor-transistor logic ("TTL"), and emitter-coupled logic ("ECL"). A variation of ECL is positive ECL ("PECL"). Each of these digital electronic varieties has a different format for output and input voltages corresponding to the HIGH and LOW states.
Referring to FIG. 1, the typical output and input voltages for HIGH and LOW states for TTL, +5 volt CMOS and ECL are shown. The shaded areas above the line are the ranges within which the output voltages for the LOW and HIGH states fall. The arrows associated with each of these ranges indicate the typical LOW and HIGH output voltage levels. The cross-hatched areas below the line are the ranges within which the input voltages for the LOW and HIGH states fall. The arrow below each line indicates the typical threshold voltage dividing what is viewed as a LOW state from a HIGH state.
As can be readily understood from FIG. 1, if an output stage of a circuit is configured for the format of one of these popular varieties of digital electronic, it, in many cases, would not be compatible for any of the other formats. This also extends to other digital electronic formats that are not as popular.
Therefore, it is desirable to have a single logic output stage that is capable of selectably generating output digital signals having one of a plurality of formats, thereby obviating the need to construct a logic output stage for each format.